1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
2. Background Art
Along with microminiaturization of semiconductor integrated circuits, MIS semiconductor elements are getting more and more miniaturized. ITRS (International Technology Roadmap for Semiconductors), 2003 Edition, predicts that EOT (Equivalent Physical Oxide Thickness) of gate insulators will be reduced to 0.7 nm or less in 2010 or later. To prevent leak current while employing so thin gate insulators thinned to this level, the use of highly dielectric metal insulators (High-k films) such as ZrO2 and HfO2 will be indispensable.
High-k films have many vacancies, for which oxygen atoms diffuse fast. Therefore, if a High-k film is formed as a gate insulator on a silicon substrate, SiOx will be produced along the interface between the silicon substrate and the gate insulator. In addition, when a gate electrode of polysilicon is formed on the gate insulator, SiOx will be produced along the interface between the gate insulator and the gate electrode. SiOx increases EOT and invites crystalline defects.
To cope with this problem, there is a proposal to use a MIS structure including a germanium substrate thermodynamically weaker in chemical reaction with High-k films than silicon and a metal as the gate electrode (see C.O. Chui et al., IEEE Electron Device Letter, 23, 473(2002)).
Although germanium is thermodynamically less reactive with High-k films than silicon, it forms a GeO2 layer, which is low in thermal stability and low in dielectric constant, along the interface between such a High-k film and the germanium substrate. The GeO2 layer invites an increase of EOT (see D. Chi et al., 3. Appl. Phys., 96, 813(2004)). Furthermore, easy-to-oxidize metals undesirably react with High-k films. Oxygen atoms from the High-k film diffuse into an easy-to-oxidize metal. Therefore, if a metal is used as the gate electrode, a metal oxide layer is formed along the interface between the High-k film and the gate electrode. When oxygen atoms diffuse from the High-k film, vacancies in the High-k film increase. This results in increasing the leak current and degrading the reliability.
Here is a need for a semiconductor device having a gate insulator suppressed in thermal reaction with a semiconductor layer and in thermal reaction with a gate electrode, thereby reduced in leak current and EOT.